In my copending U.S. patent application Ser. No. 08/862,481 filed May 23, 1997 now U.S. Pat. No. 5,864,111, I have disclosed a method and device for controlling pipe welding which is particularly useful in the welding of pipes using orbital welding techniques, such as are used in the construction of plants. Such pipes or pipelines run long distances in modern plants, particularly manufacturing plants for electronic components and semiconductors, for carrying a variety of process gases through the plant. In addition, many fabrication shops use such welding apparatus for producing manifolds, headers and fittings involving numerous welds of pipe. Generally the construction of pipelines involve the welding of numerous angles and fittings to stainless steel piping, typically ranging from 3/16 inch up to about 2.5 inch in diameter. Because of the length of the pipelines, the requisite purity of the gases to be conveyed, and the number of welds in a typical pipeline, a very consistent, high quality weld is required. Such welding techniques generally utilize an orbital welding apparatus such as is shown in U.S. Pat. Nos. 5,136,134 and 5,223,686 to Benway, et al. With such an apparatus, the welding electrode orbits around the pipe joint during the welding, while an inert purge gas flows through the pipeline.
Because of the length of the pipeline, and the fact that each weld increases the length of the pipeline and thus the volume of the pipeline, the purge gas flow characteristics change as the pipeline progressively increases in length. Moreover, as a weld is being completed, factors such as gravity and fluctuations in purge gas pressure can affect the quality and uniformity of the weld. For these reasons, controlling the pressure of the inert purge gas through the pipeline during the welding is therefor a very important aspect of the welding process for achieving high quality, uniform welds.
Thus, prior patents such as U.S. Pat. No. 5,396,039 to Chevrel disclose systems for welding pipelines using an orbital welding method with high purity argon as the purging gas flowing through the pipeline. U.S. Pat. No. 5,425,492 to Thode discloses another orbital welding process using an inert gas to protect the weld site from oxidation. This latter patent suggests adjusting the flow rate of the inert gas in response to pressure changes near the weld.
Still another system is disclosed in U.S. Pat. No. 5,304,776 to Buerkel et al, utilizing a continuously variable pressure and vacuum within the pipes of the inert gas flowing through the piping network, the pressure varying according to the position of the orbiting welding electrode. In addition, the spacing between the welding electrode and the molten weld puddle is also varied, in order to compensate for gravitational effects on the molten metal during the welding process.
As discussed in my prior patent application, the pressure and flowrate of the inert gas are two important variables which have a great influence on the quality of the welds produced in orbital welding systems. For example, a given flow rate must be maintained in order to adequately protect the weld site against oxidation. The actual flow rate is a function of several factors, but primarily the size of the pipe being welded. In addition, the pressure of the inert gas must be maintained within narrow limits during the welding. While the flow may be set at the desired rate and will remain essentially constant, control of the pressure is more difficult for several reasons. Since each weld increases the length of the pipeline, the internal volume of the pipeline also increases. In addition, when different lengths of pipe are added to a pipeline, the internal volume of the pipeline again increases. With different diameters of pipe, different volumes are encountered.
In order to maintain a uniform sweeping of a weld site by the purge gas, the flow rate of the purge gas can vary from about 5 cfh (cubic feet per hour) to almost 600 cfh, depending on the size of the pipe, but the gas pressure must stay within a rather narrow, low pressure range of about 1-5 inches of water.
Leakage from the piping network during the welding is also a significant variable which causes variation in the pressure of the purge gas. For example, during the welding some gas is allowed to leak at a butt joint, but as the weld progresses around the pipe, the joint gradually closes, and the leakage gradually diminishes until the weld is completed, at which time the leakage stops. This in turn causes a gradual increase in the pressure in the pipeline.
In the past, in a typical welding setup, to help maintain a constant pressure in the piping network, a restrictor is used at the end of the piping network, remote from the weld, and a sensitive pressure gauge is used to set and monitor the pressure. The gauge is connected to the pipeline downstream of the weld by a T-fitting connection to the pipeline, and the operator monitors changes in the pressure in order to be able to adjust the flow of the purge gas as the monitored pressure changes. Unfortunately, changing of the flow does not occur quickly enough in may cases to compensate for changing pressures. Thus, as a weld progresses around a pipe, the gradual increase in the gas pressure could increase to the point where the weld would "blow out" and cause a rejection of the weld. Even if the weld does not blow out, if the pressure rises sufficiently, the quality of the weld will be reduced.
My prior patent application describes a method and apparatus allowing a very precise control of the purge gas, by causing at least a portion of the flowing purge gas from the pipe to flow through a back pressure regulator which is capable of continuously compensating for back pressure variations in the pipeline, without allowing a pressure buildup or reduction in the pipe during the welding. This is done by means of a T-fitting connection which conducts a portion of the flowing purge gas into a receptacle containing a liquid such as water, in such a way that the purge gas continually bubbles through the water. The height of the liquid column is adjustable to control the back pressure.
The present invention relates to an improved receptacle for use in that process, and for containing the liquid which is used to create the backpressure, and through which the liquid flows.
Because of the high purity required for the welds being produced, the materials, the apparatus, and the techniques used to produce such welds are often kept in "clean rooms" in order to prevent or minimize impurities from getting into the pipelines or the welds. This in turn requires extraordinary care through all stages of the process and with all apparatus in use.
Because the apparatus disclosed in my prior application is a water column, there is always the possibility that the device will be simply tipped, or accidentally knocked over. When this happens, water would be allowed to escape from the receptacle, causing a contamination of the clean room environment. For this reason, great care must be exercised to minimize or avoid spillage. However, preventing even the possibility of leakage is complicated by the fact that the gas bubbling through the water must be free to escape to the atmosphere, and the top of the water column itself must be at atmospheric pressure. If the device were sealed or closed, the problems of prior art welding systems would not be overcome, since pressure would then be allowed to build up in the device, and the back pressure in the pipeline would gradually increase.
Accordingly, a primary object of the present invention is to provide an improved device for regulating the back pressure of a flowing purge gas in an orbital welding system.
Another object of the invention is to provide an improved device for regulating the back pressure which minimizes or prevents leakage from the device in the event that it is knocked or tipped over.
Still a further object of the invention is to provide an improved device for regulating the back pressure of a flowing purge gas, which both provides an adjustable backpressure and prevents or greatly minimizes leakage or spilling upon tipping over.
Yet a further object of the invention is to provide a relatively spillproof or spill resistant, liquid filled back pressure regulator which is open to the atmosphere at the gas discharge outlet.